fix(setup): materialize the pinned package-runner cache before the first client start (#7032)

* fix(setup): materialize the pinned package-runner cache before the first client start

Package runners key their package cache by the requested spec, so the
entry installed for the @latest invocation that ran setup cannot serve
the pinned rea-agents@<version> registration. The first client start
installed that entry inside the client's MCP startup deadline and timed
out (three consecutive 30 s timeouts observed in one client log; a
controlled cold install with every tarball already cached took 22 s).

Setup now materializes the pinned entry once after applying
registrations: best-effort, bounded by a timeout, launched from a
neutral working directory, and non-fatal on failure. The setup result
carries the prewarm summary, interactive output prints a warning when
the step fails, and direct launcher registrations keep skipping it.

Fixes #7027

* fix(setup): gate prewarm to applied registrations and supervise the runner tree

Address review findings on the prewarm step:

- Gate the prewarm on at least one applied client registration
  (configured_<client>), so an empty plan or a skill-only run never
  installs into the package-runner cache; the step now only follows
  registrations the user approved in this transaction.
- Replace execFileOutput with a supervised spawn: on timeout the whole
  runner tree stops through cleanupWindowsProcessTree on Windows and a
  process-group kill on POSIX, because killing only the direct child
  leaves npm holding the inherited output pipes past the deadline.
- Emit started and completed progress events around the prewarm so an
  interactive cold install no longer looks like a frozen setup.

* fix(setup): forward terminal interrupts to the detached prewarm runner

A detached POSIX package-runner group no longer receives the terminal's
Ctrl-C, so setup could exit while the runner kept installing in the
background. The supervised launch now registers SIGINT and SIGTERM
forwarding for the duration of the prewarm: the runner tree is stopped
through the same family cleanup as the timeout, and the default
termination is restored afterwards. An interrupted launch reports a
failure reason instead of a hang.

src/cli/interactiveSetup.ts

@@ -100,6 +100,8 @@ export const renderInteractiveSetupResult = (
       .map(([client]) => clientDisplayNames.get(client) ?? client);
     const readyClients = readyAgentClients(result, changedClients);
     renderReadyCapabilities(result, readyClients, environment);
+    if (result.prewarm?.status === "failed")
+      writeLine(`!  ${result.prewarm.reason}`);
     outro(
       changedClients.length === 0
         ? "REA is ready for local app analysis."